Fix fatal error when $wgValidateAllHtml is enabled and apache_request_headers() does not exists

Changed apache_request_headers() to headers_list() since the latter is always available

Change-Id: Ia52f5a0d12db0680aa03b57dd45c974291e88af9
Merge "Fix fatal error when $wgValidateAllHtml is enabled and apache_request_headers() does not exists"